Many communication systems require a low profile aperture antenna that can be easily conformed to an existing structure such as the skin of an aircraft, inside a moving vehicle, or concealed beneath a surface, and that can provide a steered beam. In the past, monolithic microwave integrated circuit (MMIC) or other electronically scanned or steered planar phased arrays have been used for such applications because they provide a low profile aperture. The usual reasons why a consumer may choose an electronic phased array include the phased array's ability to provide high speed beam scanning and meet multi-beam/multi-function requirements.
Unfortunately, there are several disadvantages associated with implementing an electronically steered phased array. The most notable disadvantage is that electronically steered phased arrays are very costly since the amplitude and phase at each point in the aperture is controlled discretely. The active circuit elements required to operate such an array are complex, costly and susceptible to failure. Due to this high cost, commercial exploitation of electronically steered phased arrays has been limited. Rather, the use of electronically steered phased arrays is basically confined to military and other government programs where minimizing costs are not necessarily of the highest priority. However, for most commercial applications mitigating costs is a high priority when implementing antennas or other communication devices.
An alternative to electronically steered phased array antennas is a mechanically steered scanning antenna utilizing admittance plates. These admittance plate antennas produce a directional beam by differentially rotating two, co-axial, flat admittance plates relative to each other. Some admittance plates are designed to efficiently pass incident, circularly-polarized, radio frequency energy (i.e. a beam) through them while imparting a phase shift to the beam. The direction of travel of the beam is typically changed from its original direction to a new, different direction when the phase of the beam is changed. Although, admittance plate antennas provide a viable option to antenna consumers requiring a low profile, relatively low-cost antenna capable of steering a beam, admittance plate antennas have several shortcomings associated therewith. For example, admittance plate antennas can only produce a small phase shift to the beam over the passband of the beam. This means that admittance plate antennas cannot steer a beam to extreme angles relative to the antenna. In order to steer the beam to wider angles, multiple admittance layers are used for each plate. Moreover, some admittance plate antennas are polarization dependent, meaning that the admittance plate can only impart phase changes to beams having a particular polarization. Thus, while admittance plate antennas provide a low cost alternative to electronically steered phased arrays, the admittance plate antennas sacrifice much in the way of performance.
Still another type of antenna capable of providing a steered beam is a mechanically steered directional antenna, such as a mechanically steered dish. However, such antennas have a relatively high profile, and are therefore unsuitable for applications requiring a low-profile antenna.
For these reasons, there exists a need for a method and apparatus that provides a relatively inexpensive, reliable, and low profile antenna displaying high quality beam steering capabilities.